In the manufacture of optical waveguide fiber, it has become increasingly important to produce low-cost, high quality fiber. A typical design for optical fiber uses a core region of silica doped with refractive index modifiers surrounded by a cladding region of undoped silica. The outside diameter of the cladding is typically 125 .mu.m, with the core region of a singlemode fiber being about 8-10 .mu.m in diameter and the core region of a multimode fiber being about 50-62.5 .mu.m in diameter. The equipment used to produce the doped silica is generally more complex than the equipment used to produce the undoped silica because of the need to control the dopant concentration, although both types of equipment must be capable of producing high quality product which is substantially free of impurities.
One method of reducing the production cost of optical fiber is to use a "two step" process for the manufacture of preforms which are drawn into fiber. The process comprises the following steps:
a. produce a core soot preform which contains all of the doped silica core region and, preferably, a small portion of the cladding region; PA1 b. dehydrate and consolidate the core soot preform; PA1 c. draw core canes from the consolidated core soot preform; PA1 d. overclad the core cane with undoped silica which forms the remainder of the cladding region; PA1 e. dehydrate and consolidate the overclad soot preform; and PA1 f. draw the consolidated overclad preform into fiber and apply protective coating.
Because the equipment used to produce the overclad preform can be substantially less complex than the equipment used to produce the core preform, the overall cost of manufacturing fiber can be reduced. A process as described above can be employed advantageously to produce both singlemode and multimode fiber, although the economic advantages are greater in the case of singlemode fiber because of the greater proportion of cladding region relative to core region in the resulting fiber. The two step process is compared to a "one step" process in which a soot preform containing both the core and complete cladding regions is deposited, dehydrated and consolidated, and drawn into fiber.
In fiber manufacturing processes, measurements of optical properties are generally made after the fiber is drawn. These measurements of optical properties provide information which is used to adjust the manufacturing process steps to improve or maintain optical characteristics of the fiber. However, in the two step process noted above, this "feedback" can be delayed by the extra steps involved in making the core and overclad preforms (steps c through e) as compared to the more traditional fiber process of making a single preform which forms the entire core and cladding regions of the fiber when drawn. Also, if optical property problems can be detected prior to overcladding the core cane, dehydrating and consolidating the overclad preform, drawing and coating the fiber, the significant cost of these steps can be avoided, and only the defective core cane or preform is discarded. Additionally, the information obtained by measuring the core preform or cane can be useful in adjusting the subsequent processing steps to provide fiber with particular characteristics.
It is desirable, therefore, to have methods and apparatus for measuring the optical properties of either consolidated preforms and/or core canes drawn therefrom. The methods and apparatus used should generally be nondestructive and provide quick and easy analysis of the preforms and/or canes. There are a number of commercially available analyzers which are capable of providing quick and accurate measurements of core cane. These measurements are used by manufacturing personnel to reject defective canes before subsequent processing as well as to predict the correct amount of overcladding to apply to the core cane in the subsequent step of forming an overclad soot preform. Representative of these devices are models P-102 and P-104 Preform Analyzers, which are available from York Technology Ltd. of Chandler's Ford, Hampshire, England. These devices typically operate at wavelengths of about 632 nm, although wavelengths in the range of 632 to 900 nm have also been used.
The key optical parameter for multimode fiber is the bandwidth. Bandwidth, which is specified at specific operating wavelengths, is a measure of the data capacity of a fiber. The larger the bandwidth at a given wavelength, the greater the data capacity. The bandwidth of a multimode fiber is highly dependent on the refractive index profile of the fiber. In order to achieve the highest bandwidth, the refractive index profile must be closely controlled to obtain, as nearly as possible, the optimum profile shape.
The refractive index profile of a fiber is determined primarily during the step of producing the core soot preform (step a in the process outline above), although there are other process steps after producing the core soot preform which can have some impact on the refractive index profile of the resulting fiber.
Numerous methods have been proposed for analyzing the refractive index profile of optical waveguide preforms. These include axial as well as transverse methods. Axial methods are most applicable to profiling fibers rather than preforms because of the requirements to have flat perpendicular ends on the fiber or preform or, in the case of axial interferometric methods, a thin slice cut from the fiber or preform. Transverse methods are generally nondestructive which is of particular consequence when analyzing preforms. For a summary of both axial and transverse profiling techniques, see, W. J. Stewart, "Optical Fiber and Preform Profiling Technology", IEEE J. of Quantum Elec., vol. QE-18, no. 10, pp. 1451-1466, October 1982.
Transverse profiling techniques generally require sophisticated algorithms to reconstruct a refractive index profile. See, for example, W. J. Glantschnig, "How Accurately Can One Reconstruct an Index Profile From Transverse Measurement Data?", J. of Lightwave Tech., vol. LT-3, no. 3, pp. 678-683, June 1985. Also, the various transverse methods suffer from spatial resolution limitations and require complex apparatus to provide transverse measurement data. See, Stewart, pp. 1461-1464.
The measurement methodology for transverse profiling techniques is described with reference to FIG. 1(a). Wavefront 1 is passed transversely through sample 2, which can be either a preform, cane or fiber. Because of differential phase delays caused by the varying refractive index along the sample radius, indicated by arrow 3, wavefront 1 becomes distorted wavefront 4 after passing through the sample. Various transverse profiling methods measure either the phase delay, as indicated by the curved section 5 of distorted wavefront 4, or, alternatively, the ray bending, as indicated by arrows 6.
One transverse profiling technique known as the beam deflection technique is described with reference to FIG. 1(b). A focused beam of light 11 is transversely incident on sample 12 at a distance x from the sample axis 13 and passes through sample 12. Beam 11 is deflected as it passes through sample 12 because of the refractive index of sample 12. Beam 13 is detected by detector 14 at a distance y from the sample axis. Beam 11 is then indexed across sample 12 as shown by arrow 15. The deflection data, y, is collected as a function of x, the position of beam 11 from the radius of sample 12. Mathematical deconvolution of the deflection data as a function of beam position is used to reconstruct the refractive index profile of the sample. Collection of the data and the subsequent mathematical manipulation is typically done using a computer.
A significant impediment to the accurate determination of the refractive index profile of preforms and core canes is the structure of index striae in a consolidated preform. Index striae result from refractive index variations within layers as the preforms are formed. Index striae can result in preforms made by any of the techniques used in the commercial production of optical fibers, including the outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), plasma-enhanced chemical vapor deposition (PECVD) and vapor axial deposition (VAD) methods. The striae can occur in singlemode, multimode or dispersion-shifted preforms and canes.
In the OVD method, the refractive index profile is determined by the temperature at which soot is deposited as well as the flow rates of the refractive index dopants, such as GeO.sub.2, during the soot deposition step. The soot is deposited along the length of a rotating target in successive layers. Further details on the OVD method may be found in M. G. Blankenship et at., "The Outside Vapor Deposition Method of Fabricating Optical Waveguide Fibers", IEEE J. of Quantum Elec., vol. QE-18, no. 10, pp. 1418-23, October 1982.
The index striae cause a complicated deflection pattern to be generated when known methods of profiling preforms and core canes are used. The degree to which the striae will adversely affect the ability to analyze a cane or preform depends on: (a) the spacing of the striae, and (b) the amplitude of the striae. While the striae are present in varying degrees in preforms and canes made by all known manufacturing processes, the striae appear to have a greater effect in preforms and canes manufactured by the OVD method and is particularly problematic for multimode preforms and canes made by the OVD method.